The present invention relates to thermally stabilized polyetherimides. More particularly, the invention concerns polyetherimides which have been stabilized against loss of strength and other mechanical properties upon heating and aging.
Polyetherimides are unique polymers which exhibit superior physical and chemical properties, including high heat resistance, exceptional strength and excellent processability. These polymers can be used as wire coatings and are particularly suited for injection molding applications. Because of their excellent heat resistance and high glass transition temperatures, these polymers are often used in high performance applications, where they are exposed to elevated temperatures and high mechanical loads. Although polyetherimides are excellently suited to such applications, there is a continuing need for high performance polymers, particularly those that can be formed into useful articles by the relatively convenient and inexpensive technique of injection molding.
As will be more fully described herein, polyetherimides may be made by a variety of synthetic procedures. Presently, the preferred procedures for the commercial production of these polymers involves a step known as "melt polymerization." In the melt polymerization process, a mixture of an aromatic bis(ether anhydride) and an organic diamine (or a prepolymer thereof) is introduced into a heated extruder, wherein the reactants are melted. The reaction of these components occurs in the molten state, and the molten product is extruded through a die, after which it is cooled and chopped into pellets that can be conveniently utilized in injection molding applications. The processing temperatures associated with these production techniques can be quite high. For example, temperatures on the order of 400.degree. C. and higher are employed in the extruder. Moreover, because of the high glass transition temperatures of these resins, injection molding must generally be conducted at similarly high temperatures.
Because of the elevated temperatures involved in the production and processing of polyetherimides, stabilizing these polymers against thermal degradation or deterioration has proved to be difficult. Typical polymer-stabilizing compounds are, themselves, unstable or poorly stable at these elevated temperatures. Inorganic stabilizers or organometalic stabilizers can have good stability, but suffer from the disadvantage that they are often incompatible with polyetherimides. These compounds cause discoloration and streaking of the resulting products, which can be disadvantageous for many applications.
A wide variety of substances, including polymers, are stabilized against heat degradation or deterioration by employing small amounts of antioxidants. Numerous antioxidants have been developed and are employed for stabilizing various polymers. Such antioxidants include secondary aryl amines, hindered phenols, organic phosphites, and thioesters, to name a few. For a review of the factors involved with the stabilization of polymers using antixodants, see Paolino, P. R., Plastics Design and Processing, May 1980.
Important considerations for the selection of an antioxidant system include the compatibility of the antioxidant with the polymer system, the thermal stability of the antioxidant compound over the temperature ranges to which the polymer will be exposed, the volatility of the antioxidant compound, the effectiveness of the antioxidant in stabilizing the polymer, and the effect the antioxidant has on various physical properties of the polymer. In general, polymer stabilization using antioxidants has heretofore been limited to relatively low temperature applications. For example, polyethylenes, polypropylenes, polyurethanes, polyacrylates, and the like are manufactured and processed at temperatures lower than about 400.degree. F. and usually below about 300.degree. F. The antioxidant systems used to stabilize such polymers have generally been found unsatisfactory for the stabilization of polyetherimides, because of one or more of the considerations recited above. In particular, such antioxidants have met with limited success for polyetherimides because of the high processing temperatures employed. Antioxidants tend to be volatile and/or thermally unstable at these temperatures.
Accordingly, there is a continuing need for thermally stabilized polyetherimides whose useful temperature ranges are even broader than those presently available.